This invention relates to an electro-optic measurement device for optically measuring an electric signal in an electronic component, which device comprises a radiation source for supplying an optical radiation beam, a sensor comprising an electro-optic crystal, an optical system which is arranged between the radiation source and the crystal in order to focus the radiation beam in a radiation spot on a first side of the crystal which faces the component to be measured, and a detection system for converting a change of phase in the radiation beam, generated by the electro-optic crystal due to birefringence induced in the crystal into an electric signal, the crystal being mounted on a transparent carrier means.
The described electro-optic measurement device is known from the publication "Non-contact picosecond electro-optic sampling utilizing semiconductor laser pulses" by S. Aoshima et at., SPIE, Vol. 1155 Ultrahigh Speed and High Speed Photography, Photonics and Videography '89, pages 499-510. The measurement device described therein comprises a sensor in the form of an electro-optic crystal which is positioned in the vicinity of an electric signal to be measured and originating from an electronic component. The electro-optic crystal is mounted on a transparent substrate by way of a base surface. The electric field generated by the signal changes the birefringence of the crystal. As a result, a light-beam, for example, originating from a laser and applied to the crystal will incur a change of phase. The birefringence and hence the electric signal responsible for the birefringence can be measured by detection of said change of phase.
The sensitivity of this device is highly dependent on the distance between the electro-optic crystal and the component to be measured. This distance should be as small as possible in order to ensure that the electric field generated by the signal at the point of the component to be measured is as large as possible at the area of the crystal. However, the component to be measured exhibits irregularities so that said distance cannot be arbitrarily small. In the cited publication in "SPIE Vol. 1155 Ultrahigh Speed and High Speed Photography Photonics and Videography, 1989, pp. 499-504, it is proposed to increase the electric field in the crystal by introducing a medium, preferably a liquid medium having a comparatively high dielectric constant, between the crystal and the component to be measured. However, the cited publication already notes that the effect of this medium is inadequate in the case of high signal frequencies. Moreover, in the known device the electric fields originating from signals at points in the vicinity of the component point to be measured may extend as far as the electro-optic crystal, notably if the geometrical distance or the effective distance reduced by the dielectric medium is too small, so that these stray fields influence the birefringence of the crystal and cross-talk is liable to occur. This cross-talk effect occurs especially during measurements performed on integrated circuits comprising a large number of components per unit of surface area.